There are numerous methods and systems for detecting radiation. In one type of detector, photocathodes are used in conjunction with microchannel plates (MCPs) to detect low levels of electromagnetic radiation. Photocathodes emit electrons in response to exposure to photons. The electrons can then be accelerated by electrostatic fields toward a microchannel plate. A microchannel plate is typically manufactured from lead glass and has a multitude of channels, each one operable to produce cascades of secondary electrons in response to incident electrons. A receiving device then receives the secondary electrons and sends out a signal responsive to the electrons. Since the number of electrons emitted from the microchannel plate is much larger than the number of incident electrons, the signal produced by the device is stronger than it would have been without the microchannel plate.
One example of the use of a photocathode with a microchannel plate is an image intensifier tube. The image intensifier tube is used in night vision devices to amplify low light levels so that the user can see even in very dark conditions. In the image intensifier tube, a photocathode produces electrons in response to photons from an image. The electrons are then accelerated to the microchannel plate, which produces secondary emission electrons in response. The secondary emission electrons are received at a phosphor screen or, alternatively, a charge coupled device (CCD), thus producing a representation of the original image.
Another example of a device that uses a photocathode with a microchannel plate is a scintillation counter used to detect particles. High-energy particles pass through a scintillating material, thereby generating photons. Depending on the type of material used and the energy of the particles, these photons can be small in number. A photocathode in conjunction with a microchannel plate can be used to amplify the photon signal in similar fashion to an image intensifier tube. The detector can thus be used to detect faint particle signals and to transmit a signal to a device, e.g., a counter, that records the particle's presence.
One problem with the use of photocathodes in conjunction with microchannel plates is that the electrostatic fields that accelerate electrons toward the microchannel plate also accelerate positive ions toward the photocathode. Positive ions are common in most image intensifier tubes due to impurities in the tube, including the microchannel plate and the phosphor screen. These impurities can include positive ions and chemically active neutral atoms that can become positively charged. When the positive ions collide with the photocathode, they can cause both physical and chemical damage. This greatly shortens the useful life of the photocathode and the device in which it resides.
The problem of positive ions can be overcome to some extent by placing an ion barrier film on the input side of the microchannel plate. The film serves to block the positive ions from reaching the photocathode. The barrier has the unfortunate side effect of reducing the transmission of electrons. This interference reduces the signal to noise ratio of the detector, e.g., an image intensifier tube.
An alternative method of overcoming the problem removes impurities from the components of an image intensifier tube in order to reduce the number of positive ions impinging on the photocathode. Less positive ions equates to less damage to the photocathode and a longer life for the image intensifier tube.
The aforementioned methods do not provide a means of "hardening" the photocathode itself in order to increase the photocathode's resistance to damage from positive ion collisions. A hardened photocathode would be valuable in that it would have a longer lifetime than a normal photocathode. The hardened photocathode could be used alone or in combination with the impurity removal procedures mentioned above to greatly prolong the lifetime of an image intensifier tube or other device. Consequently, what is needed is a method and system for detecting radiation that incorporates a hardened photocathode and that increases the resistance of the photocathode to ions.